Linda, I looked at several sites for the Chicago area. This one
http://web.extension.illinois.edu/ve...row_potato.cfm is saying potato planting dates from April 1 - 15th.
I'm not superstitious, but planting on April 1 might mean you plant a potato and it grows up being a bull nettle...
I wish I could answer your question better. The farthest north I have visited is Northern Arkansas.